1. Field of the Invention
The present invention relates to an encoder for measuring movement of an object, and more particularly, it relates to a photoelectric encoder for measuring movement of a diffraction grid, wherein a luminous flux having an interference characteristic is applied to the diffraction grid by which a plurality of diffracted light beams are generated, an interference fringe is formed by effecting interference of the diffracted light beams and the movement of the diffraction grid is measured by photoelectrically converting the interference fringe to an electric signal.
2. Related Background Art
Recently, in the field of precision machines such as numerically controlled machine tools, semiconductor printers and the like, there has been a need to provide a precision measuring device which can measure the movement of a moving object in the order of 1 .mu.m or less (submicron).
For example, a conventional rotary encoder as shown in FIG. 1 includes a disc 35 connected to a rotatable shaft 30 of a motor and the like, a so-called main scale 31 having a plurality of light permeable areas and light shield areas arranged alternately and equidistantly on a peripheral portion of the disc, a so-called fixed index scale 32 positioned to face the main scale and having a plurality of light permeable areas and light shield areas arranged alternately and equidistantly in correspondence to those of the main scale, and photoelectric means, 33, 34 comprising light emitters 33 and light receivers 34 arranged in registration with each other to interpose the scales 31, 32 therebetween. In this rotary encoder, when the main scale on the disk is rotated, a pulse signal is obtained in dependence on widths of the light permeable area and light shield area. Thus, by analyzing frequency of the pulse signal, the fluctuation or variation of the rotational speed of the rotatable shaft 30 can be detected; or, by counting the number of the pulses of this signal, the amount of rotation of the rotatable shaft 30 can be measured.
However, when such a photoelectric rotary encoder is used with a machine tool, if an electric failure causing the extinction of a light source occurs, the output signal from the encoder disappears.
In this case, if the control system of the machine tool cannot detect the abnormality of the encoder immediately, there will arise an overdrive of the motors in the machine tool to cause damage of the machine tool.
Further, in other conventional encoders wherein the movement of a diffraction grid is measured by photoelectrically converting an interference fringe formed by overlapping diffraction light beams obtained by the diffraction grid, as disclosed in U.S.P. No. 3,726,595, 3,738,753 and 4,629,886 and the like, the intensity of light of the interference fringe does not often reach a predetermined value due to smudging and/or misalignment of optical parts for defining a plurality of light paths of the diffracted light beams to create the interference fringe, or due to damage and/or smudging of the diffraction grid itself, as well as due to the above-mentioned electric failure of the light source and the like.
Accordingly, in these conventional encoders, the cause of the damage of the machine tool with which the encoder is associated and/or the electric failure of the encoder is further increased. Thus, there is a need to overcome these disadvantages.